An emerging field of research suggests that many disease conditions and environmental factors may accelerate cellular age (AKA epigenetic age) relative to chronological age. Epigenetic aging is thought to play a direct role in age-related illnesses and its biomarkers alone can significantly predict mortality. One of the most valid and versatile epigenetic biomarkers compares predicted epigenetic age [based on quantification of DNA methylation (DNAm)] to actual chronological age in order to observe evidence of accelerated epigenetic aging (i.e., DNAm Age). Another putative molecular signature indicative of epigenetic age may also be telomere length. Telomeres shorten with age and as a consequence of chronic disease and/or exposure to various drugs and toxins. Studies have found evidence of accelerated epigenetic aging associated with many disease states, but these methods have yet to be applied to drug abuse. Heroin use disorder is a chronic disease with numerous acute and enduring harmful consequences. Although much is known about the neurotoxicity of long-term heroin use, little is known concerning its genotoxicity. This study will be the first to assess for age-associated DNA methylation patterns and telomere length among heroin users (n = 125), in comparison to non-abusing controls (n = 125). DNA samples isolated from peripheral leukocytes will be obtained from male and female participants of African ancestry, who confirm heroin as their drug of choice. A non-abusing control group of participants will be recruited and matched based on age, race, sex, socioeconomic level and smoking status. Multiple clinical inventories and assessments will be used to minimize the effects of other variables that may also affect the biomarkers of interest (e.g., lifetime adversity, early-life stress, and autoimmune or inflammatory diseases). Standard Illumina methylation assay will be used to quantify DNA methylation levels by calculating the ratio of intensities between methylated (signal A) and un- methylated (signal B) sites. DNAm Age will be determined using the prediction method based on the methylation levels of 353 CpGs. To determine telomere length, the ratio of telomere copy number over beta-hemoglobin copy number will be obtained and multiplied by the length of the telomere amplicon. These data will allow us to determine the degree of accelerated epigenetic aging attributable to heroin use by comparing cases to controls, and also assess accelerated cellular aging as a function of duration of heroin use. This area of research is innovative as it may identify novel, quantitative biomarkers of drug harms and reveal previously unknown genotoxic effects of drug abuse.